Topographic retinotectal connections have been demonstrated in many animal species. The aims of this project are to study, mainly by electron microscopy but also with the Golgi and Cajal reduced silver stains, the formation and growth of the optic pathway from the retina to the tectum in the chick embryo. Specific sites for study are: (1) the retina including the optic fissure, (2) several regions along the optic nerve and tract, (3) optic chiasm, (4) tectal surface, and (5) terminal connections in the tectum. In general, the overall aims of this study are to determine: (1) if any factors are evident which cause retinal ganglion cell axons to become grouped into fascicles and grow into the optic fissure, (2) the interaction between fascicles from both eyes at the chiasm, and along selected sites of the optic tract, and (3) the formation of permanent synaptic connections in the tectum. Material will be examined at various times between 2 days of incubation and hatching.